1. Field of the Invention
The present invention relates generally to incandescent lamps for motor vehicles and, more specifically, to a system and method for operating designed-for low voltage incandescent lamps with a high voltage source for a motor vehicle.
2. Description of the Related Art
Electrical loads in vehicles such as motor vehicles are increasing to a point that in the future the present twelve volt (12v) system may not be sufficient. There is a consensus of opinion that higher system voltage is needed to meet future electrical loads. The voltage being proposed is forty-two volts (42 v) with an open circuit battery voltage of 36.8 v. Going to a 42 v system would necessitate redesigning the electrical loads to a higher voltage or the use of a direct current (DC) to DC converter to convert the higher voltage to a lower 12.8 v.
Redesigning electrical loads for a higher voltage is generally not a problem for motors, relays and other components in a motor vehicle. However, the incandescent lamps are an exception. Redesigning the incandescent lamps for three times the voltage would necessitate going to nine times the resistance for the same wattage lamp. That would mean that the cross-sectional area of the lamp filament would have to be reduced by a factor of three and the length of the filament would have to be increased to three times. A disadvantage of the smaller filament diameter is that the incandescent lamp has a much shorter lamp life due to the effect of mechanical vibrations causing a smaller diameter lamp filament to break sooner. From the point of the life and durability of the incandescent lamp for motor vehicle applications, a lamp design for 12.8 v or better yet, one designed for 6.4 v, would be preferred by motor vehicle manufacturers in terms of lower warranty cost and higher customer satisfaction.
One attempt to overcome this disadvantage in a 42 v system for a motor vehicle is to use a DC-to-DC converter to convert the system voltage of 42 v down to the 12.8 v. In the motor vehicle, the total lamps or lighting load varies from around 300 watts to 600 watts. The cost of the DC-to-DC converter of this power rating is high. Also, there is an issue of additional size and weight of the DC-to-DC converter. As a result, it is desirable to operate or drive incandescent lamps with a high voltage source directly in a motor vehicle. It is also desirable to operate or drive 12.8 v incandescent lamps with a 38.4 v source in a motor vehicle. It is further desirable to enable incandescent lamps to be operated from a higher voltage source without the use of a DC-to-DC converter in a motor vehicle. Therefore, there is a need in the art to provide a system and method for operating incandescent lamps with a high voltage source in a motor vehicle, which meets these desires.